The present invention relates to a pneumatic booster mounted on a brake apparatus for a vehicle such as an automobile, and a vacuum control valve used therefor.
Generally, in a brake apparatus for an automobile, a pneumatic booster is provided so as to increase a braking force. In this pneumatic booster, an intake vacuum of an engine is introduced into a vacuum chamber of the booster, and, due to a differential pressure relative to atmospheric pressure, a thrust force is generated in a power piston provided in the booster, thus increasing a force for operating the brake apparatus.
In a pneumatic booster of this type utilizing an intake vacuum of an engine, in a driving condition where the intake vacuum of the engine is low, for example, immediately after cold start, a sufficiently high negative pressure (level of vacuum) cannot be obtained, thus lowering a servo power. Therefore, proposals have been made to employ a pneumatic booster utilizing an ejector, so as to increase a negative pressure introduced into the vacuum chamber (see Unexamined Japanese Patent Application Public Disclosure Nos. 59-50894 and 60-29366).
However, the above-mentioned conventional pneumatic booster utilizing an ejector has the following problems. Air is always supplied through the ejector to a portion of an intake pipe of an engine downstream of a throttle valve, even when the negative pressure in the vacuum chamber of the booster is high. Therefore, appropriate engine control is impeded by a change in air/fuel ratio. Further, because the ejector is operated utilizing an intake vacuum of the engine, a negative pressure cannot be generated when the engine is stopped, and decreases a servo power.
In view of the above, the present invention has been made. It is an object of the present invention to provide a pneumatic booster in which an operation of an ejector is appropriately controlled in accordance with a driving condition and a stable negative pressure can be always supplied to a vacuum chamber. It is another object of the present invention to provide a vacuum control valve used for this booster.
In order to solve the above-mentioned problems, the present invention provides a pneumatic booster in which a negative pressure in an intake pipe of an engine is introduced into a vacuum chamber of a booster body, to thereby obtain a servo power. The pneumatic booster comprises an ejector and a control valve. The ejector has an air outlet connected through the control valve to the intake pipe, an air inlet open to the atmosphere and a vacuum pick-up port connected to the vacuum chamber. In response to the negative pressure in the vacuum chamber, the control valve opens when the negative pressure does not reach a predetermined level, and closes when the negative pressure reaches the predetermined level.
With this arrangement, the control valve is open until the negative pressure in the vacuum chamber of the booster body reaches a predetermined level. The ejector is operated as a result of the negative pressure in the intake pipe and a negative pressure is supplied from the vacuum pick-up port into the vacuum chamber. When the negative pressure in the vacuum chamber reaches the predetermined level, the control valve is closed and an operation of the ejector is stopped, and a negative pressure is directly supplied from the intake pipe to the vacuum chamber. Therefore, when the negative pressure in the vacuum chamber is sufficiently high, an operation of the ejector is stopped, thus minimizing an effect of the ejector with respect to an air/fuel ratio of the engine.
The present invention further provides a vacuum control valve which is incorporated into the above-mentioned pneumatic booster and effects the above-mentioned function.
In the pneumatic booster and the vacuum control valve, the ejector includes a nozzle, a diffuser disposed downstream of the nozzle and a suction opening disposed between the nozzle and the diffuser. The nozzle and the diffuser may be formed as a single body.
By this arrangement, it is possible to obtain a compact structure which enables a high negative pressure to be produced from a low negative pressure resulting from the operation of the engine.